The invention relates to a device for measuring the lubrication of surfaces rolling or sliding relative to each other and lubricated with a lubricant, equipped with an oscillator generating an AC voltage signal by means of which the capacitance occurring between the lubricated surfaces is measured.
In a known device, the said capacitance is picked up in a capacitive bridge to which the AC voltage signal of the oscillator is supplied. Besides, a measuring circuit is connected to the bridge, delivering an output signal coinciding with the capacitance to be measured. In practice, this known device is found not to function satisfactorily, in part because, with film thicknesses small compared to the surface rugosity, far higher capacitances are measured than the values theoretically to be expected.
The object of the invention is to provide a device of the kind referred to above, in which the disadvantages of the known device are eliminated.
According to the invention, the device is for that purpose characterized in that the said capacitance is firstly grounded and secondly connected by way of a condenser to the output of the oscillator, which output is further connected to a first input of a demodulating circuit, while the voltage occurring across the capacitance is supplied to a second input of the demodulating circuit, the output of the demodulating circuit being connected to a processing unit, which derives a signal corresponding to the thickness of the lubricant film as well as the percentage contact time during a certain interval of measurement from the output signal of the demodulating circuit.
In this way, a device is obtained by means of which simultaneously the thickness of the lubricant film and the percentage contact time during a certain interval of measurement can be measured. Thus it is possible at low values of .lambda., where .lambda. is the ratio of the thickness of the lubricant film to the surface rugosity, to correct the measured thickness of the film for the measured percentage contact time.
According to the invention, the demodulating circuit may consist of a multiplier and a low-pass filter connected to the output of the multiplier. It is thus brought about that the demodulating circuit delivers an output signal depending exclusively on the thickness of the lubricant film and on time, whence the processing unit can derive the desired measurements in a simple manner.
According to the invention, the processing unit is provided with a circuit forming the reciprocal of the output signal of the demodulating circuit, as well as the first means connected to the output of the reciprocal circuit and delivering an output signal corresponding to the thickness of the lubricant film and an output signal corresponding to the capacitance occurring between the lubricated surfaces.
Advantageously, the processing unit may be provided with second means converting the measured percentage contact time into a correction signal supplied to a second input of the reciprocal signal, in such manner that with increasing percentage contact time, the output signal of the reciprocal circuit decreases. In this way an automatic correction of the thickness measurement of the lubricant film is obtained, thus providing an accurate result of measurement.
Preferably, the voltage occurring across the capacitance present between the lubricated surfaces is delivered to the demodulating circuit by way of a band-pass filter tuned to the oscillator frequency, the oscillator being likewise connected to the demodulating circuit by way of a band-pass filter tuned to the oscillating frequency. In this way any possible network interference deriving from the system to be tested is suppressed. The band-pass filter for the oscillator signal serves to maintain the correct phase relationship between the input signals of the demodulating circuit.